A compressor of a vehicular air-conditioning system has obtained its drive power from a vehicle engine. In such a case, in general, the compressor drive torque is estimated and the estimated drive torque is used to control the engine output. As a result, it is designed so that even if the drive torque of the compressor changes, the engine speed will not fluctuate. When obtaining the drive power for the compressor from the vehicle engine, suitable estimation of the compressor torque is an important issue.
For example, Japanese Unexamined Patent Publication (A) No. 2006-272982 discloses the following type of torque estimating means.
At the initial period of startup of a compressor, a startup stage torque estimating means estimates the torque of the compressor. At the steady state, a stable stage torque estimating means estimates the torque of the compressor. For this purpose, the above two torque estimating means are successively switched after compressor startup, so that the torque can be suitably estimated in accordance with the stage after compressor startup.
In this regard, the rising behavior of the drive torque in the transitory state such as right after startup of the compressor differs depending on heat load such as the state of the high pressure side pressure and low pressure side pressure etc. Due to this, in the estimation of the torque of a compressor such as in Japanese Unexamined Patent Publication (A) No. 2006-272982, if estimating the rise in the drive torque right after startup of the compressor without considering the heat load, depending on the conditions, there will be the problem of a discrepancy between the estimated drive torque and the actual compressor drive torque.